The present invention relates to magnetic fluids. Magnetic fluids are defined as Newtonian liquids that retain their fluidity in the presence of an external magnetic field. These fluids comprise stable colloidal suspensions of magnetic particles in such liquid carriers as hydrocarbons (kerosine, heptane, etc.), silicones, water, and fluorocarbons.
While the term "ferrofluid" was used to designate a magnetic colloid in which the dispersed phase is a magnetic ferrous material, the more general term "magnetic fluid" is preferred because these fluids may contain ferromagnetic particles other than iron--i.e. cobalt, nickel, gadolinium, and dysprosium. They may also contain ferrimagnetic substances other than magnetite (Fe.sub.3 O.sub.4) or maghemite (.gamma.-Fe.sub.2 O.sub.3). Examples are the magnetic ferrites of manganese, cobalt, nickel, copper, and magnesium. Further discussion of magnetic fluids, their properties and their uses may be found in an article by S. E. Khalafalla published in Chemical Technology, Volume 5, September 1975, pp. 540-546 and in the bibliography therein.
In preparing magnetite for use in magnetic fluids, one usually starts with an aqueous solution of ferric and ferrous salts from which the magnetite particles are precipitated. Accordingly, the preparation of a water-based magnetic fluid appears desirable. Several methods have been proposed for the preparation of such water-based systems. In one system, dodecylamine is used as a dispersing agent. While this material can be used to prepare a water-based magnetic fluid, the fluid is not dilution stable. When diluted, flocculation occurs. Although these dilution sensitive fluids are suitable for some applications, they are unsuitable for other applications, such as mineral beneficiation, in which dilution occurs. Another water-based system, utilizing petroleum sulfonate dispersing agent, is described in U.S. Pat. No. 4,019,994. That fluid, however, is also not dilution stable.
It is an object of the present invention to provide a dilution stable water-based magnetic fluid and a further object is to provide a method of making same.